Work Package 14 (Brumsack/Pahnke, Hillebrand, Blasius)

"The fate of selected trace metals (Mo, Mn, Fe, REE) and their isotopes (Fe, Nd) in coastal waters of the Southern North Sea"

Broader background of the proposed research project

It has been known for long, that many trace metals are actively involved in bio-cycling processes (see review by Bruland et al., 2014) and serve as essential micro-nutrients for plankton growth, whereas others are not (conservative elements). The role of most elements is – in a broader sense – quite well understood for the deep open ocean, but the role of coastal or near shore settings for the cycling of these elements in general still requires scientific attention. When approaching the coast, the chemical signature of seawater becomes increasingly superimposed by fluvial and estuarine processes, by subterranean groundwater discharge and interaction with the sedimentary porewater system (Beck et al., 2012; Rouxel et al., 2008). As a consequence, trace metal signatures of coastal waters are seemingly much more affected by seasonality than open ocean sites. Recently it was found that so called conservative elements like Mo may exist in different species indicating also different reactivity (Wang et al. 2009). For the Southern North Sea only few and randomly taken trace metal data are published in the open literature. Most of these data were not generated under rigid contamination control, except for the data set published by Achterberg et al. (2003), where the processes mentioned above could for the first time be shown for the elements Cu and Co. Significant changes in Mo concentration were detected by our own group during specific time intervals associated with the breakdown of plankton blooms in the Southern North Sea (Kowalski et al., 2013).

Outline for the proposed PhD research project

Water samples will be taken contamination-free on transects from the open North Sea towards the coast during different seasons and, preferentially, at the onset, maximum and ending of plankton blooms. After filtration and pre-concentration water samples will be analyzed for trace metals (Mo, Mn, Fe, REE) by HR-ICP-MS. Selected samples will be investigated for their stable Fe and Nd isotopic composition. In a second set of samples the fluvial contribution from major rivers (Elbe, Weser, Ems) will be chemically characterized. Information on the contribution from sediment pore waters and the subterranean estuary system will be gained from previous or ongoing projects. Additional information will be experimentally determined by using pure and mixed plankton cultures from the North Sea resolving different processes (e.g. change in Mo-species: plankton derived or pore water source). The major goal of the research project will be to generate a seasonally resolved trace metal, species and metal isotopic data set for the Southern North Sea to document and model the impact of biological processes (plankton blooms, changing redox state of bottom sediments) on water column signatures. We hypothesize that a) coastal processes contribute to a significant fraction to the trace element budget of the open ocean, b) the breakdown of plankton blooms significantly affects some of the so called conservative trace metals in the coastal ocean, c) some of the unexpected behavior of so called conservative elements (e.g. Mo) is related to a species shift. This project will contribute to the data basis of WP 13. The temporal and spatial variability of trace metals and selected isotopes will be related to the abundance of planktonic species and interpreted in an ecological context provided by WP 13.